function  out=f_get_spiral(F_centers,Norder,nturn)
for jk = 1:length(F_centers)
    r_nonlinear(jk) = sqrt( F_centers(jk,1)^2 +  F_centers(jk,2)^2 );
end

xx = 1:length(F_centers);
P = polyfit(xx,r_nonlinear,Norder)
xx_fine=1:0.01:length(F_centers);
fitted_r=0;
% fitted_r = P(1).*xx_fine.^2 + P(2).*xx_fine + P(3);
for i=1:length(P)
    fitted_r=fitted_r+P(i).*xx_fine.^(Norder+1-i);
end
[x_spiral y_spiral]=f_plot_spiral([0 0], [F_centers(1,:); F_centers(end,:)],nturn,fitted_r);
out.x_spiral=x_spiral;
out.y_spiral=y_spiral;